1. Field of the Invention
The present invention relates to an image reading apparatus that optically acquires an image of an original, and an image forming apparatus provided with the image reading apparatus.
2. Description of the Related Art
Recently, with the advent of new technology, it has become possible to read image data with high speed. Moreover, the frequency of the reading operation clock has also been increased in association with the speed-up of the reading operation. With the increase in the frequency of the reading operation clock, however, it has become essential to take countermeasures against electromagnetic interference (EMI) due to the regulations on electromagnetic wave intensity (FCC, VCCI, etc.).
An SSCG (spectrum spread clock generator) is generally employed as an effective countermeasure against EMI. This technique involves integrally planarizing the EMI spectrum by frequency-modulating an operation clock frequency within a fine range over a long period.
When the SSCG is in an image reading apparatus, the effect of modulation of the SSCG disadvantageously appears on an image. More specifically, the modulation of the SSCG acts on the driving of a charge-coupled device (CCD) device that is typically used as a photoelectric conversion device and on the operation clock of an analog/digital converter (A/D) whereby an offset level of an image disadvantageously gets synchronized with the period of the SSCG modulation and fluctuates, although slight. This phenomenon becomes visible as a streak PP, a horizontal or oblique streak, on the image (see FIG. 8).
A technology for removing a fluctuation component from an image signal so that the horizontal streak will not appear is known in the art. For example, Japanese Patent Application Laid-open No. 2008-118366 discloses a related art.
In the technology disclosed in Japanese Patent Application Laid-open No. 2008-118366, an image reading apparatus is provided with a function of converting an incident light to an analog image signal with a photoelectric conversion device and digitizing and outputting the analog image signal with an analog-to-digital converter. The image reading apparatus includes a fluctuation removing circuit that drives the photoelectric conversion device by a frequency-modulated clock and superimposes a signal with a fluctuation amount the same as but in inverted phase to the fluctuation of the analog image signal on the image signal according to the change in frequency of the clock (see FIG. 9).
A signal in which amplitude and a phase of a correction signal to be superimposed on the image signal are previously set is generated and is superimposed on the image signal. As an implementation example thereof, it is configured to cause a bias current of an emitter-follower connected transistor to change.
However, because the amplitude and the phase of the correction signal to be superimposed on the image signal are previously set, the conventional technology may lead to such a situation that it cannot deal with a case in which offset fluctuation due to the effect of the SSCG is not uniform in each device.